Over the past 2 decades it has been revealed by satellite observations
that loss of grounded ice in West Antarctica has been increasing, while
the floating ice shelves have been thinning. When an ice shelf thins it
becomes less effective at constraining the discharge of grounded ice,
yet understanding of the climate processes diving mass discharge is
limited. In this study ice shelf height data obtained from 4 satellite
altimeter missions (1994 to 2017) were used to show that there was a
direct link between the variability of ice shelf height in the
Pacific sector of
the Antarctic and changes in the regional atmospheric circulation that
were driven by the ENSO. In
the sector from the Dotson Ice Shelf to the Ross Ice Shelf this link was
found to be the strongest and elsewhere the link was weaker. In years of
intense El Niño the increase in height resulting fromaccumulation of snow exceeds the decrease in height that results
from basal melting, though there is a decline in the mass of the ice
shelf as the loss of basal ice exceeds the ice gain by snow, which has a
lower density than the ice that is lost. It was demonstrated by the
results of this experiment that there was a substantial response of the
ice shelves of the Amundsen Sea to global and regional climate
variability, with rates of height and mass loss on interannual time
scales that can be comparable to the longer-term trend, with a
significant fraction of basal melting changes being offset by mass
changes from surface accumulation. It is implied by this that the
variability of the ice shelf height and mass will increase as the
interannual atmospheric variability increases in a climate that is
warming.

As a result of insufficient understanding of the processes that cause
mass loss from the ice sheets in
Antarctica and
Greenland projections of global changes in sea level over the next
century are not certain (Church et al., 2013). The West Antarctic Ice
Sheet (WAIS), which is mostly grounded below sea level (Fretwell et al.,
2013), is therefore a marine ice sheet, contains enough ice above
flotation to increase the global sea level by more than 3 m.
It is suggested by ice shelf
models (Favier et a;l., 2014;
Joughin, Smith & Medley, 2014; DeConto & Pollard, 2016) that as glaciers
and ice streams respond dynamically to mechanisms of internal
instability (Joughin, Smith & Medley, 2014; Weertman, 1974) mass loss
will accelerate. The ice loss from the WAIS that has been predicted on
timescales of decades to centuries is about 1 m of global sea level
equivalent, and full loss of ice sheets within a few millennia (Joughin,
Smith & Medley, 2014; DeConto & Pollard, 2016).

In the Amundsen Sea sector of the West Antarctic Ice Sheet the
acceleration of the loss of grounded ice has been attributed to a
reduction of backstress as the fringing ice shelves are thinning and
their grounding lines are retreating (Joughin, Smith & Medley, 2014;
Rignot et al., 2014). The rapid and sustained thinning of the ice
shelves in the Amundsen Sea (Paolo, Fricker & Padman, 2015; Pritchard et
al., 2012) appears to be caused by increasing wind-driven flow of warm
Circumpolar Deep Water (CDW) into the ocean cavities beneath the ice
shelves, which enhances the basal melting (Pritchard et al., 2012;
Jacobs et al., 2011). It is suggested by models (Thoma et al., 2008;
Steig et al., 2012) and limited observations (Dutrieux et al., 2014;
Jacobs et al., 2013) that ice shelves might be responding to changes in
the circulation of the Circumpolar Deep Water on interannual timescales.
A paucity of time series of ocean observations in the continental shelf
offshore of the Amundsen Sea ice shelves and the sub ice cavities limits
this ability to confirm this hypothesis, which led Paolo et
al. to seek indirect measures
of the sensitivity of the mass change of the ice shelf to large scale
climate variability.

In the Southern Hemisphere the SAM is a major variability of climate,
which influences precipitation and temperature patterns from the
subtropics to Antarctica (Marshall, 2003; Abram et al., 2014). The
southern spring and summer are the times when SAM is usually strongest.
The effect of ENSO in Antarctica is influenced by the phase of SAM, with
the strongest response of the Pacific sector to ENSO occurring when SAM
is weak or in opposite phase (Fogt, Bromwich & Hines, 2011), i.e., with
the combinations of La Niña/SAM+ and El Niño/SAM- strengthening the
anomalies of atmospheric circulation in the mid- to high latitudes.

The Amundsen Sea Low is a low pressure system that is persistent within
the Amundsen Sea and the Bellingshausen Sea, and it plays a dominant
role in the determination of the regional-scale pattern of the
circulation of the atmosphere across West Antarctica (Raphael et al.,
2016; Hosking et al., 2013; Turner et al., 2017). The Amundsen Sea Low
determines the wind anomalies in all seasons through changes in strength
(i.e., central pressure) and position, strongly influencing snowfall,
temperature distribution and the conditions of the sea ice near the ice
shelves of the Amundsen Sea (19). Variations in the position and
strength of the Amundsen Sea Low are driven by variability of the
tropical Pacific ocean-atmosphere (ENSO) and fluctuations in the
pressure of the Southern Hemisphere (Turner et al., 2017; 17-19). The
central pressure of the Amundsen Sea Low tends to be lower during
positive SAM conditions and La Niña years, and higher during higher El
Niño years (Turner et al., 2017; 17,31,32),

In this study Paolo et al.
show that the height of the ice shelf and its mass changes in the
Pacific sector, in the Amundsen Sea sector in particular, are correlated
with interannual variability in regional atmospheric and oceanic
circulation driven by ENSO.

Climate controls on change of the ice shelf

As well as ENSO and the Amundsen Sea Low, there are other regional modes
of interannual variability, e.g., the zonal wave three (Raphael, 2004)
may also contribute to changes that are observed in the Pacific sector
of ice shelves, even though their correlations with
δh(t)
might be negligible; e.g., SAM. These other modes interact with ENSO and
with each other to drive complex patterns of changes in ice shelf height
that cannot be explained solely by a linear combination of climate
modes.

Studies (Turner et al., 2017; Turner et al., 2017; Raphael & Hobbs,
2014; Clem & Fogt, 2013) that correlated ENSO tropical forcing with
climate indicators of the Pacific sector, such as the strength of the
Amundsen Sea Low, the extent of sea ice and temperature of the Antarctic
Peninsula, have found that correlations with ENSO are significant for
some seasons though not with others, with in some cases sign reversals
for the correlations from season to season. In the Amundsen Sea Low
changes in the strength, location and extent, which all display large
interannual variability, result in each distinct ENSO event yielding
different responses in individual ice shelves, depending on their
location. Assessments of the height of the ice shelves by Paolo et
al. for the Amundsen Sea
sector reflect the unique environmental conditions of the strong ENSO
period of 1997-2000, during which there were exceptionally large changes
in the Amundsen Sea Low between the El Niño and La Niña cycles. The
strong El Niño event of 2015-2016, in contrast, was characterised by
remarkably large changes in sea temperature at the surface of the
tropical ocean, did not impact ice shelves in the Amundsen Sea in the
same way. It is suggested by Paolo et
al. that this probably
resulted from the interaction of El Niño with a positive SAM during the
development phase, a northwards displacement of the ASL relative to
previous strong El Niños (Fogt, 2016) and the absence of a strong La
Niña.

The increased basal melting that is associated with the onshore flow of
the Cold Deep Water and upwelling along the coast as there is an
intensification of the westerly wind stress is the dominant effect of El
Niño on the mass of the ice shelf in the Amundsen Sea (Thoma et al.,
2008; Steig et al. 2912; Dutrieux et al., 2014). This basal mass loss
anomaly on interannual time scales, relative to the loss of mass trend
(Paolo, Fricker & Padman, 2015), is offset partially by an increase in
snowfall. This increase in precipitation is consistent with the northern
wind anomaly that occurs during El Niño events, that possibly includes
local uptake of moisture from the coastal ocean due to a reduction in
the concentration of regional sea ice.

According to Paolo et al. the
reversed pattern of meridional wind anomalies between the Amundsen Sea
and the Bellingshausen Sea-Antarctic Peninsula (BS/AP) sector is
consistent with the SMB for the WAIS as well as with the surface mass
balance that has been obtained from satellite-gravity measurements over
the Amundsen Sea and Antarctic Peninsula sectors (Sasgen, 2010). During
El Niño in the Amundson Sea sector precipitation increases though it
decreases in the BS/AP region and the opposite occurs during La Niña.
The response of the Amundsen Sea Low pressure system to ENSO and SAM
(Turner et al., 2017; Turner et al., 2017; Fogt, Bromwich & Hines, 2011)
is the principal mechanism that affects the local winds, with the
Amundsen Sea Low changing consistently in its intensity between El Niño
and La Niña years. A weaker response to ENSO was shown by the Pine
Island Glacier Ice Shelf, which is thinning rapidly, which is consistent
with the weaker, and in some cases reversed, atmospheric and sea ice
anomalies in Pine Island Bay relative to the broader Amundsen Sea.

It was shown by Paolo et al.
how the height and mass of the ice shelves in the Amundsen Sea sector of
the WAIS is affected by the ENSO. The response in height consists of the
combined effect of 2 opposing processes, both of which intensified
during El Niño events: accumulation of surface snow and basal melting
that is driven by the ocean. The result is an overall increase in
height, though not a loss of mass, as the ice that is lost from the base
is of higher density than that of the snow being deposited on the
surface. Given that the atmosphere of the Earth warms it is expected
that there will be increases in precipitation and frequency of ENSO
events (Cai et al., 2014),
the results obtained by Paolo et
al. imply that interannual variability of ice shelf height and mass
will also increase, stressing the need to quantify accumulation at the
surface relative to basal melting to project future changes in ice
shelves in Antarctica.